1. Field of the Invention
This invention relates to a polycrystalline silicon film and a method of manufacturing the same. More particularly, it relates to a polycrystalline silicon film of low resistance for use in, for example, the interconnections and gate electrodes of a MOS integrated circuit (IC), etc., and a method of manufacturing the same.
2. Description of the Prior Art
Polycrystalline silicon films for use in, e.g., the interconnections of various semiconductor integrated circuits and the gate electrodes of MOS.multidot.ICs should desirably have the lowest possible resistances in order to enhance the performances of the ICs etc.
In reducing the resistance of a polycrystalline silicon film, it is common practice to employ a method in which the polycrystalline silicon film is doped with an impurity such as phosphorus, arsenic and boron by thermal diffusion, ion implantation or the like.
With the prior-art impurity-doping method resorting to thermal diffusion, ion implantation or the like, however, the resistivity of the polycrystalline silicon cannot be made below approximately 4.times.10.sup.-4 .OMEGA..multidot.cm because the maximum concentration of the impurity to be activated in the silicon is determined by a thermodynamical solid solubility.
More specifically, although it is possible to introduce the impurity into the polycrystalline silicon film in large amounts by, for example, ion implantation, the quantity of the impurity which can enter the crystal lattice points of the polycrystalline silicon film is determined in dependence on the temperature. Therefore, when after the ion implantation the polycrystalline silicon film is annealed at a high temperature in order to remove the distortion of the crystal having occurred due to the ion implantation, the situation as to quantity of the impurity is as follows. While the polycrystalline silicon film is at high temperatures, the impurity atoms in very large amounts enter the crystal lattice points. However, as the temperature lowers, the impurity atoms are gradually released from the crystal lattice points. At last, only that quantity of the impurity which is governed by the solid solubility remains in the crystal lattice points.
The resistivity of the polycrystalline silicon film is determined by the quantity of the impurity existing in the crystal lattice points (activated impurity atoms).
Accordingly, even when very large amounts of impurity atoms are introduced by the ion implantation, the quantity of the impurity atoms which can enter the crystal lattice points cannot be increased for the reason described above. It has therefore been extremely difficult to lower the resistivity. Moreover, when semiconductor devices are mass-produced in a factory, the attained resistivities of polycrystalline silicon films are still higher. This has also been a serious problem in practical use.
In order to employ a polycrystalline silicon film in various usages such as the interconnections and gate electrodes of multifarious ICs having high densities of integration, the resistivity of the polycrystalline silicon film needs to be made below approximately 4.times.10.sup.-4 .OMEGA..multidot.cm. To this end, a more effective impurity-doping or -introducing method to replace the prior-art method has been desired.